Cytogenetic analysis has been used to identify tumor susceptibility genes in human cancers. The objective of this project is to employ molecular cytogenetic methods to characterize and target genetic regions in a transgenic mouse model for identification of genes that are important in the development of hepatocellular carcinomas. The cooperation of the c-myc oncogene with the growth factor transforming growth factor alpha (TGFalpha) in development of liver tumors in transgenic mice has previously been demonstrated. In the current study we analyzed the ploidy and karyotype of myc, TGFalpha, parental control and the double transgenic myc/TGFalpha hepatocytes at three weeks of age when the liver is normal and at 10 weeks when the myc/TGFA-alpha liver is dysplastic with basophilic foci. The chromosome number and morphology of the TGFA-alpha, the myc and the myc/TGF-A-alpha hepatocytes was essentially normal at three weeks of age. Eighty percent of the 10 week hepatocytes from myc/TGFalpha mice were aneuploid and 32% had chromosomal breakage. Statistically significant breakage was observed in six different chromosomes. Breakage at band A5 and at the border of bands C4/5 of chromosome 1 was observed. Fragile sites on chromosome number 4 were most frequent in the middle of the chromosome at bands C2 and C6. Chromosome number 6 was fragile at band F2. The region of chromosome number 7 near the linkage group containing Ha-ras at band B5 and D1 was frequently broken and involved in translocations. Chromosome 12 was broken at bands D1 and D3. The breakage sites on chromosomes 1, 4, 7, and 12 correspond to sites of tumor susceptibility genes. Although there was no consistent change in copy number, recurrent translocation between chromosomes 1, 4, 7, 12 and 19 were also observed. These studies demonstrate that the development of dysplasia and basophilic foci in the liver of myc/TGFalpha transgenic mice is correlated with aneuploidy and chromosome breakage. The specific fragile sites indicate genetic regions that are altered during early stages of hepatocarcinogenesis. Due to the conservation of genetic lineage groups between mouse and human, the identification of genetic alterations in the mouse will aid in the identification of novel tumor susceptibility in the human.